Drainage Permit Applications ChecklistApril 2014
MnDOT Drainage Permits Checklist
Purpose of the MnDOT Drainage Permit
MnDOT Metro District regulates activities that impact its drainage systems and its MS4
regulated area. The purpose of the Drainage Permit is to protect State of Minnesota
investment in infrastructure including but not limited to roadways, storm water treatment
basins, ditches and storm sewer systems. Excess storm water and/or sediment laden storm
water added to MnDOT’s drainage systems leads to degradation of these assets. Negative
impacts include but are not limited to: sediment deposition, loss of flood storage capacity
and also loss of hydraulic conveyance capacity. These impacts may cause premature
flooding of the road surface and/or erosion damage on State right-of-way.
Technical Requirements of the MnDOT Drainage Permit
The permit applicant shall demonstrate that offsite runoff coming to MnDOT drainage
system and/or right-of-way will not increase as a result of the proposed project. This is
quantified as a “no increase in discharge” criteria for the 2-year, 10-year and 100-year
storm events. Compliance is demonstrated by applying hydraulic/hydrologic software
models. HydroCAD and XPSWMM are the approved models to compare the pre and post
project discharge values. Typically, HydroCAD is sufficient to model most proposed
projects. However, XPSWMM may be required if the project contains extensive storm
water pipe systems connected to MnDOT storm sewer or if HydroCAD cannot in
MnDOT’s judgment effectively model pressure flow, complex junctions and/or
backwater effects that are present. The 2-year, 10-year and 100-year storm events shall be
based upon Atlas 14 runoff amounts per the NOAA website.
In addition, Drainage Permit Applicants shall meet all applicable water quality treatment
requirements established by the local Watershed District(s) and the MPCA.
Permit applicants should anticipate that specific projects that seek to divert runoff to
another sub-watershed or watershed will be denied. It is MnDOT practice to avoid such
watershed diversions whenever practicable.
Submittal Requirements:
Readable/legible watershed maps that show pre and post project drainage
conditions. These two separate contour maps shall be large enough in scale so that
approximate flow paths can be determined for verifying the Time of
Concentrations used in the models. The drainage/watershed maps shall include
enough detail so that Curve Numbers used in the hydraulic models may be
verified by MnDOT.
April 2014
Surface water flow direction and storm water pipe water flow direction shall be
indicated on the pre and post project watershed maps.
Minimum recommended watershed map scale is 1” =100’. Project applications
submitted with smaller scales (e.g., 1”=500’) may be rejected and returned to the
applicant. The same would apply for project watershed maps that do not include
topographic contours or basic land use information such as the location of
buildings, pavement and “green space”. Watershed maps submitted as pdf files or
CAD files shall be readily printable at scales that allow for good readability.
Pre and post project watershed maps shall be clearly linked to the drainage
models such that the names of the sub-watersheds, ponds and drainage structures
are the same in the models as shown on the watershed maps. In addition,
watershed and sub-watershed boundaries shall be clearly shown.
Submission of the actual pre and post project HydroCAD or XPSWMM models is
required: pdf copies of the drainage model simulations are unacceptable. In the
event that the models cannot be transferred readily by electronic mail or electronic
repository site, a hardcopy CD shall be provided.
Curve numbers shall be determined per NRCS methodology and should be
modified as needed based upon detailed knowledge of soil type and specific
conditions on site. HydroCAD modeling software includes NRCS guidance for
determining curve numbers based upon land use and condition.
Time of concentration (Tc) computations and assumptions that in MnDOT’s
assessment clearly overestimate or underestimate this critical runoff parameter
will be rejected. Two common assumptions that lead to overestimating Tc
include: using the “Lag/CN” method to determine peak runoff from watersheds
that have a relatively long and/or diverse flow path, and assuming that sheet flow
occurs for a distance exceeding100 feet. Conversely, pre-project Tc shall not be
underestimated to offset post project increases in peak discharge.
Available freeboard for existing and proposed treatment ponds shall be shown on
the watershed maps as well as the normal and 100-year high water levels. All
proposed pond treatment systems along MnDOT right-of-way shall have a
minimum freeboard of 2.0 feet between the road surface and the proposed
100-year HWL.
Infiltration basins, filtration basins and ponds adjacent to MnDOT right-of-way
shall be designed to provide at least 2 feet of elevation difference between the
100-year HWL and the crest of the basin berm. The berm crest shall be at least 5
feet wide. The emergency overflow shall be lined from crest to toe of slope with
Turf Reinforcement mat or Category 6 or 7 Erosion Control Blanket.
April 2014
Best management practices (BMP’s) including infiltration/filtration sites, storm
water ponds, etc. shall be clearly labeled on the pre and post project watershed
maps.
Plan sheets submitted as watershed maps shall be evaluated as such. They shall be
readable and legible and meet all the same requirements including clear
delineation of watershed boundaries, readable map scale, and land use shown by
an aerial photo background map, or that is clearly depicted based on details on the
plan sheet or sheets submitted.
Project plan sheets relevant to the Drainage Permit are required and include:
existing site conditions, the proposed grading plan as well as proposed site
drainage system plans and profiles. The plans shall include applicable wetland
impact/mitigation features and temporary sediment and erosion control measures
for the project. In addition, erosion control blanket will be used to stabilize
disturbed area on MnDOT right-of-way unless other methods such as rip-rap
treatment are called for in the plans and approved by MnDOT.
Pond and basin special structures including weirs and orifices shall be consistent
with what is used in the HydroCAD or XPSWMM models submitted and include
relevant calculations/details.
A table summary of existing versus proposed site discharge to MnDOT drainage
system/right-of-way is required for the 2-year, 10-year and 100-year Atlas 14
rainfall events.
Post project storm water discharge to MnDOT ditches or other open channel shall
be limited to flow velocities of 6 fps or less for a 50-year Atlas 14 rainfall event.
Project discharge points that will connect to MnDOT ditch or channel shall be
located such that they do not cause erosion or conflict with the grade of the
existing ditch or channel.
Proposed access road culverts on MnDOT right-of-way shall be designed for the
10-year Atlas 14 rainfall event unless they are part of a significant drainage ditch
along the roadway in which case a 100-year or 50-year design will apply.
Direct connections to MnDOT storm system shall be avoided. Connection to open
ditch, or channel is preferred. If direct storm sewer connections cannot be
avoided, it is the applicant’s responsibility to provide a good connection typically
via a new structure. Furthermore, MnDOT offers no warranty that there will not
be a hydraulic backwater effects on the new storm line upstream that is connected
to MnDOT’s existing storm sewer.
April 2014
For all disturbed areas that sheet flow to MnDOT right-of-way and any disturbed
areas within MnDOT right-of-way, either Erosion Control Mat or Bonded Fiber
Matrix shall be used for temporary/permanent erosion control.
Silt fence shall not be used for erosion control at the proposed project site
perimeter. Rather, continuous Wood Chip or compost Sediment Control Logs
shall be implemented.
Permit applicants are encouraged to contact MnDOT Metro Water Resource Engineering
with questions/concerns. Questions posed early in the permit application process help to
avoid project delays. This is particularly true for large project Drainage Permits with
significant complexity.